Coherent optical transmission offers several advantages in long haul dense wavelength division multiplexing, DWDM, networks; link distances of thousands of kilometers are achievable with cost effective single stage optical amplifiers and without chromatic dispersion compensation in line, and improved spectral efficiency is obtainable using sub-Nyquist modulation and coding, e.g. Time-Frequency Packing.
Spectral efficiency is a key feature also in optical interconnection systems, where transmitting more capacity per line helps to decrease the number of cables, which is an issue in modern data centres, having hundreds or thousands of connected servers and storage units.
Due to modulation complexity, any spectrally efficient modulation format is very sensitive to the non-linear response of the optical modulator, which means that low driving voltage levels must be used, decreasing the signal to noise ratio tolerance of the system. Optimal settings of the voltage level depend on the link characteristics, for example, accumulated amplification noise, and require long calibration procedures in field. In meshed DWDM networks, where channel paths can be rerouted in traffic, a calibration for a first path will not be valid for a new path, leading to additional system penalty that could be avoided in principle by dynamically readjusting the modulator driving voltage.
The simplest and typical way to use digital to analogue converters, DACs, is to configure and calibrate them manually during system installation, for example, by loading one of a set of preconfigured filter shapes and setting the bandwidth. This is a very suboptimal way to use DACs, which generally makes the additional transmitter cost and installation complexity of using DACs not worthwhile, given that the optical link characteristics can change for several reasons. Rerouting an optical channel through a reconfigurable optical add-drop multiplexer, ROADM, onto a different path is a typical case. Being able to reconfigure the frequency response of a DAC would allow the channel throughput to be maximized for the new link or to maximize channel performance when the link propagation conditions changes, for example due to interfering spectrally adjacent channels being added.
An optical transmitter is described in WO 2012/163419 which has a modulator for modulating data onto an optical signal, for transmission to a receiver, and a tuning controller for tuning automatically the modulator to adjust the modulation based on a received feedback signal. The modulated signal has components at one or more constellations of points of different optical amplitudes and phases. The feedback signal indicates a distortion of one or more of the constellation points measured at the receiver. The tuning controller tunes the modulator to adjust the modulation based on the feedback signal to pre-compensate for the measured distortion.